The present invention relates to a reinforcing ply for a tire, and also to a tire reinforced by such ply. It also relates to a process for manufacturing such a ply, and to a process for manufacturing the tire which uses said ply.
A reinforcing ply for a tire is formed of reinforcement elements generally covered on either side with layers of rubber mix referred to as calendering layers, such that said elements cannot be in contact with the outside. The rubber mix used is generally a single mix of the same composition and having the same properties.
It is sometimes highly desirable for the calendering mix to be different for the same ply depending on the location of said ply within the tire, be it a carcass reinforcement ply, a crown reinforcement ply, or any other reinforcing ply. According to the teaching of U.S. Pat. No. 4,140,166, a crown reinforcement having a plurality of plies of reinforcement elements which are parallel to each other within each ply and are crossed from one ply to the next must, in order to increase the life of the tire, be provided with different characteristics in its central zone and its marginal zones. The tire of said reference, comprising a carcass reinforcement and a crown reinforcement of a plurality of plies, is characterized in that the calendering rubber mix of the crown plies has a modulus of elasticity in tension at 100% elongation of greater than 70 kg/cm2 in the median zone of the crown reinforcement, which zone is defined by the ratio of the width of said zone to the overall width of said reinforcement, and in that the calendering rubber mix of the same plies has a modulus of elasticity in tension at 100% elongation of less than 40 kg/cm2 in the remaining two marginal zones of the crown reinforcement.
U.S. Pat. No. 3,172,445 relates to a tire with radial carcass reinforcement, and the invention described consists in stiffening the bottom part of the sidewalls, by imparting thereto a lesser rigidity than that permitted by the solution currently used, that is to say, the lateral superposition of crossed cables on the radial carcass reinforcement. To this end, the stiffening of the sidewalls, in the zone between the bead and substantially the mid-sidewall, is obtained by means of at least one ply of meridian reinforcement elements coated in the calendering mix of the carcass reinforcement. In one of the embodiments described, the carcass reinforcement is formed of a single ply of radial metallic cords or cables, which is turned up around the bead wire and ends in the bead, these cords or cables being coated in the zone located between the bead and substantially the mid-sidewall with calendering layers the elasticity modulus of which is at least 350 g/mm2, which modulus is a high modulus compared with the modulus of the remaining calendering mix.
U.S. Pat. No. 3,709,277 discloses that, in certain tires with radial carcass reinforcement, the selection of the rubber connecting the reinforcement elements is an essential factor in the appearance or non-appearance of certain defects after travel of the tire. Thus the selection of a connecting rubber of low elasticity modulus promotes the separation of the carcass reinforcement cables at the end of the upturn of said reinforcement, in the beads and in the regions of the sidewall which are close to the beads, more particularly for the carcass ply which has a high upturn in the sidewalls. Furthermore, the selection of a connecting rubber of high elasticity modulus promotes the tearing of the connecting rubber between carcass reinforcement cables, along the radial lines in the top zone of the sidewalls. To obtain the best results, U.S. Pat. No. 3,709,277 advocates, more particularly for tires with radial carcass reinforcement, surrounding the cables of relatively elastic material with a layer of coating rubber of relatively high elasticity modulus, said cables being separated by an interstitial rubber mix which is of relatively low elasticity modulus in the top zone of the sidewalls and of relatively high elasticity modulus in the bottom zone of the sidewalls, the coating layer and the interstitial layer having the same moduli in the bottom zone. As for the process for ensuring that in the top zone of the sidewalls the coating rubber of the cables and the interstitial rubber between cables of one and the same ply are of different qualities, while they are identical in the bottom zone, it consists, during the production of the tire, of arranging, radially to the inside of a ply, calendered cables in a layer of rubber of a first quality, a layer of rubber of a second quality, and during the vulcanization of the tire, exerting a tension on the carcass reinforcement cables.
Be it one or the other of the prior solutions, be it for a carcass reinforcement or for a crown reinforcement, the processes used for obtaining a reinforcing ply as described do not make it possible, given the many factors affecting the quality of the semi-finished product constituted by the ply in the non-vulcanized state, to obtain the optimum structure of said ply depending on whether it is used in a carcass reinforcement or in a crown reinforcement, and even less the effective use thereof in the production of a tire.